Laffer has it right, as usual

If the "Buffett Rule" were applied as President Obama proposes, then Mr. Buffett's federal tax bill would have been $14.4 million, rather than the $6.9 million he actually paid. As a fraction of his true income, his effective tax rate would only have risen from 6/100ths of 1% to 12/100ths of 1%.

[...]

Mr. Buffett also stated in his op-ed that in his 60 years working with investors he has yet to see anyone "shy away from a sensible investment . . . even when capital gains rates were 39.9% in 1976-77." Mr. Buffett's choice of 1976-77 is prescient because the economy in 1977 was a basket case. The official Bureau of Labor Statistics unemployment rate was 7.1%, consumer price inflation was 6.7%, and the S&P 500 dropped a whopping 17% after adjusting for inflation. Indeed, 1977 is a good illustration of the type of economy Mr. Buffett's policies would deliver.

[...]

Meanwhile, the top 1% of earners saw their tax payments climb to 3.3% of GDP in 2007 from 1.5% of GDP in 1978, while the bottom 95% saw their tax payments drop to 3.2% of GDP in 2007 from 5.4% of GDP in 1978. Why would Mr. Buffett want to reverse these numbers?  Of course, cynics and die-hard progressives might object to the above evidence on the grounds that it was driven by an explosion of income gains. But that's largely the point.

http://online.wsj.com/article/SB10001424052970203462304577138961587258988.html

I've provided the highlights, but as with everything Art Laffer writes, it's best to go read the entire column for yourself.